1. Field of the Invention
The present invention is directed to an apparatus for inductively coupling a nuclear magnetic resonance signal into a reception antenna having a resonant coil arrangement that has an imaging area.
The invention is also directed to a medical intervention instrument that is connected to such an apparatus for inductively coupling a nuclear magnetic resonance signal into a reception antenna.
2. Description of the Prior Art
An apparatus of the above type is disclosed in U.S. Pat. No. 4,680,549. It is stated therein that a nuclear magnetic resonance signal from a limited area of an examination subject to be imaged can be noticeably enhanced when a separate set of coils that are closed to form a resonant circuit is arranged in immediate proximity to the area. The separate set of coils has no electrical connection to the rest of the apparatus. The precessing nuclear magnetization then induces a current in the resonant coil arrangement, this current in turn inducing an additional signal in a reception antenna that is significantly greater than the signal which is directly induced in the reception antenna by the precessing nuclear magnetization itself. The signal-to-noise ratio in the area of the image, which corresponds to the area covered by the induction coil, is noticeably improved. A detuning means in the form of two diodes connected with opposite polarities is connected to the resonant reception coil arrangement so that the excitation field is not locally modified when transmitting with a separate transmission antenna, i.e. when exciting the nuclear magnetic spins.
The improvement of the signal-to-noise ratio of the inductively coupled, signal-intensifying resonant circuit and of the resonant coil arrangement is dependent on the quality (Q) of the coil arrangement and on the angle of the imaging area of the resonant coil arrangement relative to the basic magnetic field direction. When the two reside parallel to one another, no improvement derives. This is disadvantageous when, for example, a resonant coil arrangement is to be attached to a surface of the subject to be imaged, if the surface only allows an alignment of the imaging area of the resonant coil arrangement parallel to the basic magnetic field.
On the other hand, circularly polarizing antenna arrangements are known that have two sub-antenna arrangements arranged perpendicularly to one another. Such arrangements thus have antenna characteristics perpendicular to one another that must in turn be aligned perpendicularly to the basic magnetic field direction. For example, a circularly polarizing antenna arrangement disclosed in U.S. Pat. No. 5,602,557 has two sub-antenna arrangements arranged perpendicularly to one another, with each sub-antenna arrangement comprising two saddle-shaped antenna conductors arranged opposite one another. The imaging area is located between the saddle-shaped antenna conductors.
U.S. Pat. No. 5,198,768 discloses an antenna array having dipole and quadrupole antennas.
German OS 34 29 386, corresponding to U.S. Pat. No. 4,932,411, discloses a small antenna for a nuclear magnetic resonance tomography apparatus that can be directly introduced into body organs, for example into the brain, into the liver or into the kidney, via channels. It is attached to the end of a thin plastic carrier.